The long term objective of this project is the development and marketing of easily swallowed electrodes for temporary transesophageal cardiac pacing. This "Pill Electrode" can replace cardiac catheterization in a variety of diagnostic and therapeutic applications, including initiation and/or termination of tachycardias, short term maintenance of adequate rate in sinus bradycardia, acceleration of rate to produce stress during echocardiographic or radionuclide studies of ventricular function. With transesophageal instead of catheter pacing, these become out-patient procedures with greatly reduced complexity and cost. The aim of Phase II is to conduct extensive animal and clinical studies to demonstrate safety and efficacy of the electrodes and stimulator which were designed, constructed and bench-tested in Phase I. The following hypotheses will be tested: 1) Short-term transesophageal pacing at currents and pulse-widths within the limits of the Arzco stimulator does not produce significant burning of the esophageal muscoa in dogs. 2) Transesphageal pacing with the extendible Pill Electrode results in successful left atrial capture in 95% of patients and can be used in many clinical procedures. 3) The procedure is tolerable without sedation due to the optimal electrode configuration and to the local anesthetic effect of lidocaine of similar agent delivered directly to the site of stimulation.